"stale" - it means that by the time the pool wanted to submit the block, it realized that some other miner on the network had already found it. This is similar to your local mining software reporting stale shares, where the share it submits to the pool is for a previous block and the pool had already moved on to the next block.

It's also possible that BitMinter did submit the block, but another miner submitted the block at about the same time, and their block 'won'. That would technically make BitMinter's block 'orphaned', rather than 'stale'.